1. Field of the Invention
The present invention relates to a method for driving a liquid crystal display unit, and more particularly to a method for driving an active matrix type liquid crystal display unit using a thin film transistor as a switching element.
2. Description of the Prior Art
In recent years, active matrix type liquid crystal display units are increasingly used in such devices as compact TV sets, projection TV sets, and view finders. However, such a display unit is inferior to a CRT display unit in terms of flicker, screen burning after displaying a still image, uniformity of an image displayed on the screen, gradation display capability, cost, and other factors.
The following describes a conventional method for driving an active matrix type liquid crystal display unit with reference to FIGS. 6 and 7.
FIG. 6 shows the construction of an exemplified conventional active matrix type liquid crystal display unit, while FIG. 7 shows an equivalent circuit of one pixel in the construction shown in FIG. 6. Referring to FIGS. 6 and 7, there are comprised an image signal supply circuit 21, a scanning signal supply circuit 22, an image signal line 15, a scanning signal line 16, a counter electrode line 17, a switching transistor 18, a pixel electrode 19, a capacitance 20 (C.sub.LC) of the liquid crystal material across the counter electrode 17 and the pixel electrode 19, and a parasitic capacitance 14 (C.sub.GD) across the gate and the drain of the switching transistor 18.
In the active matrix type liquid crystal display unit, a plurality of image signal lines 15 and a plurality of scanning signal lines 16 are provided intersecting each other, and at each intersecting point are provided in a matrix form the pixel electrode 19 and the switching transistor 18 which applies a voltage to the pixel electrode 19. Then a scanning signal V.sub.G is supplied from the scanning signal supply circuit 22 to the gate of the switching transistor 18 via the scanning signal line 16 to control turning on and off the switching transistor 18. Meanwhile, an image signal V.sub.S is supplied from the image signal supply circuit 21 to the pixel electrode 19 via the image signal line 15 as well as the source and the drain of the switching transistor 18. The image signal V.sub.s and a counter electrode signal to be supplied to the counter electrode 17 are applied across a liquid crystal material interposed between the counter electrode 17 and the pixel. electrode 19 to display an image.
FIG. 8 shows waveforms of the scanning signal V.sub.G, the image signal V.sub.s, and an effective voltage V.sub.B to the liquid crystal material. The scanning signal V.sub.G is a signal to be supplied from the scanning signal supply circuit 22 to the gate of the switching transistor 18 as composed of a voltage V.sub.GH for turning on the switching transistor 18 and a voltage V.sub.GL for turning off the switching transistor 18. The image signal V.sub.s is a signal to be supplied from the image signal supply circuit 21 to the pixel electrode 19 as inverted in polarity every one horizontal scanning period (1H) between a positive voltage V.sub.s.sup.+ and a negative voltage V.sub.s.sup.-. The effective voltage V.sub.B applied to the liquid crystal material is a voltage actually applied across the liquid crystal material interposed between the pixel electrode 19 and the counter electrode 17.
The following describes the operation of the liquid crystal display unit having the above-mentioned construction with reference to FIGS. 7 and 8. Assuming now that the scanning signal V.sub.GH is applied to the gate of the switching transistor 18 with the positive image signal voltage V.sub.s.sup.+ applied to the image signal line 15, the switching transistor 18 turns on to apply the image signal voltage V.sub.s.sup.+ to the liquid crystal material. When the scanning signal V.sub.GL is applied to the gate of the switching transistor 18, the switching transistor 18 turns off. Consequently, the application voltage V.sub.B to the liquid crystal material reduces by .DELTA. V due to the capacitance C.sub.GD between the gate and the drain of the switching transistor 18. The application voltage V.sub.B to the liquid crystal material is maintained by the capacitance C.sub.LC of the liquid crystal material itself until the next cycle of the scanning signal V.sub.G. In the next cycle, the scanning signal V.sub.GH is applied to the gate of the switching transistor 18 with the image signal V.sub.s.sup.- being the inverted form of the image signal V.sub.s applied to the image signal line 15 to consequently apply the image signal voltage V.sub.s.sup.- to the liquid crystal material. When the scanning signal V.sub.GL is applied to the gate of the switching transistor 18, the application voltage V.sub.B to the liquid crystal material reduces by A .DELTA. to maintain the resulting voltage. Therefore, as shown in FIG. 8, the application voltage V.sub.B to the liquid crystal material is periodically inverted in polarity. When the scanning signal V.sub.G changes from V.sub.GH to V.sub.GL, the electric potential at the pixel electrode 19 is varied by the parasitic capacitance C.sub.GD between the gate and the drain of the switching transistor 18 to vary the voltage V.sub.B applied to the liquid crystal material. The variance .DELTA. V of the voltage V.sub.B applied to the liquid crystal material is expressed by the following equation: EQU .DELTA.V=C.sub.GD .multidot.(V.sub.GH -V.sub.GL)/(C.sub.LC +C.sub.GD)
In order to compensate for the variance .DELTA. V of the voltage V.sub.B applied to the liquid crystal material, the voltage to be applied to the counter electrode 7 is preset at the central value V.sub.BC of the voltage V.sub.B applied to the liquid crystal material to symmetrically arrange the positive polarity voltage and the negative polarity voltage applied to the liquid crystal material. In other words, the above-mentioned voltage is adjusted so that the equation of V.sub.BC =V.sub.SC -.DELTA. V holds. It is noted that the value V.sub.SC of the voltage V.sub.B applied to the liquid crystal material is the central value of the image signal V.sub.S. Even though the voltage to be applied to the counter electrode 17 is preset at the value V.sub.BC, i.e., the central value of the voltage V.sub.B applied to the liquid crystal material as described above, there is no compensation for an effective direct current component which is generated by the variance .DELTA. V due to the dielectric anisotropy (the property that the dielectric constant of the liquid crystal material varies according to a voltage applied to the material) of the liquid crystal material and applied to the liquid crystal material, which has lead to the problems of flicker and screen burning occurring after displaying a still image.
In order to give solution to the above-mentioned problems, for example, Japanese Patent Application Laid-Open Publication No. Hei-2-157815 proposes to further provide a line (not shown) connected to the pixel electrode 19 via an additional capacitance (not shown) and apply a modulation signal of which polarity is inverted every one field to the line to modulate the electric potential at the pixel electrode 19 for the purpose of improving the display image quality and drive reliability as well as reducing drive power.
However, the above-mentioned construction fatally necessitates a modulation signal supply circuit having output terminals corresponding in amount to the scanning signal lines 16 and modulation signal lines other than the image signal supply circuit 21 and the scanning signal supply circuit 22 to result in increasing the circuit scale. It has been also proposed to superimpose the modulation signal on the scanning signal in the previous stage, however, such a construction requires a significantly complicated scanning signal to result in increasing the circuit scale of the scanning signal supply circuit. Furthermore, the voltage applied to the liquid crystal material fluctuates in amplitude due to the dielectric anisotropy of the liquid crystal material to result in several problems such as difficult gradation controllability of the liquid crystal display panel.